1. Field of the Invention
This invention relates to a method of supplying poly-silicon, as a raw material for producing silicon single crystal by a CZ method, to a crucible to be molten therein, a method of producing silicon single crystal using the supplying method, and poly-silicon to be used as the silicon raw material therefor.
2. Description of the Prior Art
As is well known, in the production of silicon single crystal by the CZ method, poly-silicon blocks or granules are charged in a crucible before pulling the single crystal and molten to have a given quantity of the silicon melt, and pulling of the single crystal from the melt is started.
The silicon single crystal produced by this method is rapidly becoming larger in diameter and heavier, with the result that a charge quantity of raw material is increasingly required to be larger as far as possible.
Increased crucible size and/or charge level of raw material in the crucible are measures to cope with the above requirement. Increased crucible size, however, increases power consumption for melting and makes it more difficult to control melt temperature, and hence is not a desirable measure.
Increased charge level of raw material in a crucible, on the other hand, means that the raw material reaches the level close to the upper end of the crucible. The upper end, projecting upward beyond the heater, radiates more heat and tends to be lower in temperature than the other parts. Therefore, the raw material in the vicinity of the upper end cannot be molten completely, to partly stick fast to the inner crucible wall. This portion of the raw material applies a load to the crucible wall, deforming that portion of the crucible inwardly, to cause problems, e.g., decreased serviceability of the crucible, and dislocation of the crystals resulting from deformation of the crucible. Increased level of raw material, therefore, is also not a desirable measure.
A technique of additional charge is developed to cope with the above situations. Another technique of recharge also employs the similar charge supply method. These techniques are disclosed by Japanese Patent Application Laid-Open Nos. 2-188487, 3-12385, 8-169795 and 8-310892.
The additional charge method melts the initially charged poly-silicon blocks or granules in a crucible, and then additionally charges poly-silicon rod in the melt to be molten therein. This can increase a charge quantity of raw material without increasing crucible size or raw material charge level in the crucible.
The recharge method pulls silicon single crystal from a crucible, and additionally charges poly-silicon rod in the residual melt to recover the melt quantity, thereby allowing to repeat the pulling step without exchanging a crucible. This can enhance the productivity of silicon single crystal, reduce a crucible cost, and hence greatly reduce the production cost. This technique, together with the additional charge, is considered to be essential for increasing the diameter and weight of silicon single crystal.
Object of the Invention
However, the conventional additional charge and recharge techniques involve the following problems:
Poly-silicon rod, when charged in a crucible, is brought down into the crucible while being suspended from the lower end of the pulling axis. FIG. 6 illustrates how the conventional method supports the poly-silicon rod 20. It is processed to have an annular groove, or a smaller-diameter section 22, over the outer periphery of the upper end, to be held by a molybdenum wire 50 as the supporting tool, wound on the section 22 and connected to the lower end of the pulling axis 30.
In this case, however, the wire as the supporting tool may contaminate the silicon melt, when immersed therein. It is therefore necessary to leave undissolved the upper end of the poly-silicon supported by the wire, which causes silicon loss.
The above procedure also needs, when the poly-silicon is dissolved in the melt, pulling the wire as the supporting tool together with the undissolved poly-silicon, to exchange it with the seed crystal for pulling the single crystal. Therefore, the silicon single crystal is produced by the following 8 steps:
(1) The poly-silicon rod is moved downward into the melt in a crucible.
(2) When it is dissolved, the undissolved portion and supporting tool are moved upward into a pull chamber.
(3) When the above step is completed, pressure within the pull chamber is released to the atmospheric pressure, after the gate valve between the pull chamber and main chamber is closed.
(4) The undissolved portion of the poly-silicon and supporting tool are taken out of the pull chamber.
(5) The seed chuck and seed crystal are attached to the lower end of the pulling axis.
(6) The pull chamber is evacuated again to produce a vacuum therein.
(7) The gate valve is opened, and the seed crystal is brought down into the main chamber.
(8) The seed crystal is immersed in the melt, subjected to the dipping, and pulled.
Compared with the conventional CZ pulling method, the additional charge step (1) needs additional steps (2) to (7). In particular, the step (4) for taking out the poly-silicon and supporting tool involves dangerous works because they are at high temperature, and hence needs a long cooling period. This causes the problems, lowered production efficiency of silicon single crystal by the additional charge method, and the production efficiency not increased as expected by the recharge method.
Moreover, contamination of the melt by the supporting tool is not completely ruled out, although it is kept apart from the melt.
Japanese Patent Laid-Open No. 8-169795 proposes a new supporting tool to replace the wire, and Japanese Patent Laid-Open No. 8-310892 proposes a technique to support poly-silicon rod. These methods, however, provide no solution to problems caused by the supporting tool, such as difficulty in dissolving the poly-silicon rod, lowered productivity, and contamination of the melt.
It is an object of the present invention to provide a method of supplying poly-silicon rod in a crucible without causing any risk of contamination by a supporting tool and loss due to an undissolved portion of silicon, while greatly improving the production efficiency of silicon single crystal. It is another object of the present invention to provide a method of producing silicon single crystal. It is still another object of the present invention to provide poly-silicon.
The method of the present invention for supplying a silicon raw material dissolves a poly-silicon rod, brought down in a crucible while being supported by a seed crystal, before the silicon single crystal is pulled by the CZ method.
The method of the present invention for producing a silicon single crystal dissolves a poly-silicon rod, brought down in a crucible while being supported by a seed crystal, before the silicon single crystal is pulled by the CZ method, and then immerses the seed crystal in the silicon melt in the crucible, which is pulled as the silicon single crystal by the CZ method.
Both methods of the present invention, one for supplying the silicon raw material and the other for producing a silicon single crystal, are applicable to the additional charge and recharge methods.
In the method of the present invention, whether it is applied to the additional charge or recharge method, it is necessary to pull the silicon single crystal from the silicon melt, after the poly-silicon rod is additionally supplied to the silicon melt in a crucible, and the seed crystal is immersed in the silicon melt in the pulling process.
In the method of the present invention, whether it is for supplying the silicon raw material or for producing a silicon single crystal, a seed crystal which causes no problem when immersed in the silicon melt is used to support the poly-silicon rod. Supplying the poly-silicon together with the seed crystal to totally immerse the former allows to start pulling the silicon single crystal by the CZ method no sooner than the poly-silicon is dissolved. As a result, the above-described steps (2) to (7) can be saved, to greatly improve the production efficiency of silicon single crystal. At the same time, this removes risk of contamination of the melt by the supporting tool and avoids loss of the undissolved portion of silicon.
A supporting tool of silicon which is substantially free of risk of contamination can be used to support the poly-silicon by the seed crystal. However, it is preferable, viewed from cost, to dispense with the supporting tool and directly support the poly-silicon at the lower end of the seed crystal. It can be directly supported by the seed crystal, when hung on the lower end which is made larger in diameter than the seed body. In this case, the poly-silicon is provided with a cavity in the upper end, into which the larger-diameter lower end is fit to hold the poly-silicon.
The larger-diameter section, on which the poly-silicon is hung, can be formed at the lower end of the seed crystal by machining. It can be also formed during the necking step after the seed crystal is immersed in the silicon melt in a crucible, where the larger-diameter section is formed in the neck. In this case, the seed crystal with the larger-diameter section can be reused for supplying the raw material and pulling the single crystal, when the pulled silicon single crystal is separated from the seed crystal below the larger-diameter section. It eliminates the needs for machining to form the large-diameter section and operation for changing the seed crystal upon recharging.
The poly-silicon of the present invention is a rod, to be used as the silicon raw material for the silicon single crystal pulled by the CZ method. In this case, the poly-silicon is provided with a cavity in the end, into which the larger-diameter lower end is fit to hold the poly-silicon. The poly-silicon is immersed in the silicon melt in a crucible together with the seed crystal, by which it is directly supported at the lower end of the seed crystal. This allows to start pulling the silicon single crystal by the CZ method no sooner than the poly-silicon is dissolved. As a result, the production efficiency of silicon single crystal is greatly improved. At the same time, this removes risk of contamination of the melt by the supporting tool and avoids loss of the undissolved portion of silicon.